It has been a problem of long standing in preparing monolithically integrated semiconductor components (also referred to as chips) to integrate two or more lasers capable of emitting at differing wavelengths while at the same time obtaining chips presenting planar surfaces--i.e., surfaces having the low level of relief common to semiconductor devices fabricate using planar processing. Planar surfaces exhibit a relief variance of a micrometer (.mu.m) or less.
When monocrystalline semiconductive substrates exhibit surface relief in excess of about 1 .mu.m, particularly when these relief differences are exhibited by next adjacent chip components, manufacturing difficulties arise leading to reduced yields. As the relief differences between adjacent chip elements increase, the slope of connecting surfaces of the substrate increases, shifting from a horizontal orientation toward a vertical orientation. As the surface orientations become increasingly sloped, the choice of techniques by which overlying layers, such as insulative or conductive layers, can be deposited reliably is reduced. Additionally, increased relief differences impose localized reduced radii of curvature on overlying layers. It is at these sites in overlying layers that coating non-uniformities are most common. Further, it is at the low radius of curvature layer locations that stress defects, such as those attributable to differences in thermal expansion characteristics, are most likely to occur.
It is known in the art that a laser can be formed by planar processing. Such lasers are formed by introducing along a substrate surface N and P conductivity type ions in laterally spaced regions so that an active region is created therebetween. While such lasers are ideal in terms of achieving an overall planar surface for a chip, emission efficiency of such lasers is relatively low.
For this reason lasers are normally constructed by epitaxial growth of superimposed layers. Botez, "Laser Diodes are Power-Packed", IEEE Spectrum, June 1985, pp. 43-53, provides a state of the art survey of such laser diodes as discrete elements. Positive index lasers are disclosed and schematically illustrated. Lasers can be formed with only three superimposed layers, superimposed N and P conductivity type cladding layers with an active layer intervening. Efficient lasers are typically formed with five or more superimposed layers. Botez discusses only discrete lasers and monolithically integrated laser arrays emitting at a single wavelength.
Attempts to form integrated circuits containing more than one laser having superimposed layers and with the lasers having the capability of emitting at differing wavelengths has led to constructions in which the layers of one laser are superimposed on the layers of another laser. As might be expected, this leads to exceptionally high surface relief. Lang et al U.S. Pat. No. 4,318,059 is representative of this approach.
Mito et al U. S. Pat. No. 4,318,058 discloses laterally related lasers integrated on a common chip capable of emitting at differing wavelengths. Mito et al deposits III-V compound layers on a substrate by molecular beam epitaxy while varying the III-V compound composition during an electron beam sweep across the substrate surface. An inherent disadvantage of the Mito et al process is that molecular beam deposition is slow as compared to organometallic vapor epitaxial deposition. The disadvantage of the Mito et al structure produced is that one lasing region as initially formed grades smoothly into the next laterally adjacent lasing region. Mito et al discloses a III-V compound laser formed using arsenic as group V ions and aluminum and gallium as group III ions in which adjacent lasing regions are separated by etch troughs. Thus, the overall topography of the chip is a mesa topography rather than a planar topography. Mito et al discloses an alternative planar construction using indium as group III ions and phosphorus as group V ions and resorting to implanting proton radiation insulated regions for isolation in the epitaxy between adjacent lasers. Besides the obvious laser disadvantages of this approach, no substrate surface between adjacent laser is available for integration of other, non-laser chip components. The highly doped proton radiation insulated regions are, of course, poorly suited to serving as fabrication areas for many types of circuit components.
Recognizing the difficulties of integrating waveguides such as lasers in multicomponent chips, the art has continued to investigate the properties of waveguide semiconductive materials and the manner in which such materials can be deposited. Ghosh et al, "Selective Area Growth of Gallium Arsenide by Metalorganic Vapor Phase Epitaxy," App. Phys. Lett. 45(11), Dec. 1, 1984, pp. 1229-1231, discloses the selective growth of gallium arsenide by organometallic vapor phase epitaxy on a gallium arsenide substrate partially masked by a silica layer. Tokumitsu et al, "Molecular Beam Expitaxial Growth of GaAs Using Trimethylgallium as a Ga Source," J. App. Phys. 55(8), Apr. 15, 1984, pp. 3163-3165, reports similar selective deposition employing molecular beam epitaxy. Kamon et al, "Selective Epitaxial Growth of GaAs by Low Pressure MOVPE," Journal of Crystal Growth 73 (1985), pp. 73-76, discloses the selective growth of gallium arsenide on gallium arsenide in areas not covered by silica and in "Selective Embedded Growth of AlGaAs by Low-Pressure Organometallic Vapor Phase Epitaxy", Japanese Journal of Applied Physics, 25(1), Jan. 1, 1986, pp. L10-L12, extend their previous disclosure to growing selective gallium aluminum arsenide on an etched substrate.